In the early '90s, CMOS Monolithic Active Pixel Sensors (MAPS) were invented for the detection of visible light. Because of continuous improvements in CMOS technology, CMOS sensors are becoming the dominant image-sensing device, in commercial digital cameras and scientific applications.
CMOS MAPS were also proposed and demonstrated as charged particle detectors, first for particle physics, and then for other applications, such as transmission electron microscopy. The electron energies in a transmission electron microscope (“TEM”) typically range from about 100 keV up to about 500 keV. TEMs commonly use charged-couple device (“CCD”) detectors, which are damaged by high energy electrons. To prevent damage to the CCD detector, TEM detectors include a scintillator that converts the electrons to light, which is then detected by the CCD. The intervening scintillator reduces resolution of the detector. CMOS MAPS can be used as direct detectors of charged particles, that is, the CMOS MAPS are more robust and can detect the electrons directly.
CMOS MAPS can provide a good signal-to-noise ratio, high resolution and high sensitivity, and are a significant improvement over current CCD technology using scintillators. CMOS MAPS include a thin epitaxial layer over a thicker substrate. Substantially all of the detection occurs in the epitaxial layer, which provides the detection volume. One problem with electron detectors is that electrons which are backscattered from the substrate below the detector volume can return into the detector volume, randomly increasing the signal and spreading the signal over multiple pixels. Performance of the CMOS MAPS can be improved if electron backscattering could be prevented. A known method of reducing backscattering is to thin the substrate below the detector volume, which is referred to as backthinning. High energy electrons are then more likely to pass completely through the thinned substrate without backscattering. Ninety percent of the silicon substrate of a CMOS APS does not contribute to the performance of the detector but the substrate does contribute to backscatter which reduces signal to noise ratio and blurs the image.
FIG. 1 shows Monte Carlo simulations of scattering of a 300 keV primary electron 102 in a 300 μm thick silicon sensor 104. The traces 106 represent different possible electron trajectories as determined by the simulation. The trajectory that an individual electron follows is determined by chance. Only a few of the many possible trajectories that the electron could follow are shown. A line 108 is drawn about 35 μm below the top surface, the depth to which a typical sensor may be backthinned. The sensitive volume of the sensor is the top layer 110, approximately 5 μm to 20 μm thick.
Traces 106 show the electron trajectories when a sensor is not backthinned. Traces 116 show that some of the electrons being scattered within sensor 104 back into the sensitive top layer 110. Such backscattered electrons degrade the resolution of the sensor by producing extraneous signals, many of which are away from the impact point of the primary electron 102. With the sensor backthinned to line 108, relatively few electrons are backscattered within the thinned substrate to the sensitive top layer 110, so resolution of the sensor is improved. In a thick sensor, substantial signal could be generated at a large distance from the impact point of the primary electron 102. A thin sensor can therefore greatly improve the detector performance.